Essentially the problem of bird navigation is a double one: first, 
to discover which of their senses tells birds the right direction for a 
migratory flight, and second, to find the environmental clue which pro- 
vides the information (Griffin 1944). There is much disagreement about 
the sensory mechanism responsible for bird navigation. We know that 
in birds, vision is developed to a much greater. extent than the other. 
senses. Yetit cannotdaccountfor allbird navigation, for many migratory 
flights take place at night, through‘clouds, or even in fog. 
One popular theory claims that birds are sensitive to the earth’s 
magnetic field and guide their motions accordingly (Yeagley 1947), but 
there is little supporting evidence for this idea (Thorpe, et al 1949; 
Gordon 1948). Another hypothesis advances the idea that birds can per- 
ceive the mechanical forces resulting from the earth’s rotation. There 
are enormous difficulties, however, in the way of a bird’s being able to 
feel such effects. 
Thus, there is little to encourage us in the many theories which 
suggest that some new “sense” accounts for bird migrations. But there 
are other explanations which credit bird navigation to clues perceived 
by the known sense organs. These theories, if valid, might offer more 
hope for application to human air navigation. For whatever a bird can 
see or feel might also be detectable by men. 
In this connection we must first consider a theory which undcubtedly 
has some validity, though it does not provide a full explanation. This is 
the simple idea that birds rely on contact flying. We know that birds 
have performedamazing feats in remembering Getails of things they have 
seen. B. F. Skinner, for example, has trained ordinary domestic pigeons 
to peck at a specific object in an 
aerial photograph of a small area 
of ground with food as a reward. 
He found that even four years after 
their last observation, the birds 
were able to remember it well 
enough to peck immediately at the 
appropriate point, even though they 
had séen nothing in the meantime 
which even resembled the picture. 
This type of experiment should be 
extended to include photographs of 
terrain of interest to migrating 
birds; even now, however, there is 
enough evidence to show that birds 
have highly developed ability to re- 
member the physical features ofa 
landscape. 
Another example of the im- 
portance of landmarks was indicated 
by experiments conducted in 1947 
Figure 1. Actual flight paths of 
homing gannets released at Cari- 
bou showed that birds explored in 
all directions before reaching the 
coast and presumably familiar 
territory. 
by R. J. Hock and the author. We 
were interested in the ability of 
birds to perform a sortof artificial 
migration after they are caught in 
their nests, carried in closed boxes 
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